[llvm] [RISCV] fix SP recovery in varargs functions (PR #114316)

[Sam Elliott via llvm-commits]

lenary wrote:

> > Commit 1 should be a real commit to the tree, i.e. not squashed as part of merging this PR.
> 
> I've made a separate PR with test: #114970

For future reference, this is something you can directly commit to `main` without a PR, like before we used github (it's a new test, and should be NFC and passing when committed).

https://github.com/llvm/llvm-project/pull/114316